Even with the development of electronic banking and credit cards the society still relies on cash based transactions. The risk of fake or counterfeit currency is high in such a society. Even state of the art security features have been forged. The advantage that a counterfeiter has is that he does not need to produce the exact replica of the notes but just needs to produce a simulation that is good enough to pass at least one transaction. This problem is most significant and apparent in businesses handling a large volume of paper money transfers. Environments such as casinos, currency exchanges, banks, etc., require a more automated and reliable way of preventing and detecting counterfeit currency introduction.
The best defense against forgery would be to equip the general public to distinguish a fake note from a legitimate one. The authenticity of the note has to be ensured by the receiver after proper inspection which would not need sophisticated technologies. This would prove to be one of the most efficient ways to fight forgery.
One commonly employed and easily recognizable feature is a printed image, for example in the form of a portrait of a famous person. Traditionally, such images were applied to banknotes by techniques such as intaglio printing, often including a guilloche (fine line) pattern. The result was an image having a characteristic appearance and tactile impression which, at the time, was difficult for counterfeiters to reproduce.
U.S. Pat. No. 4,980,569 discloses a verification device comprising of two optical light source/detector pairs disposed on opposite sides of a proffered currency. The source and detector pairs are arranged for transmission and reception of optical energy through the currency if the thread is not present. Also, the source and detector pairs can determine the presence of a counterfeit thread on the currency surface by checking for light reflected off the currency surface. Thus, the '569 patent provides a twofold test wherein the thread, to be genuine, must be detected under transmitted light and not be detected under reflected light. However, the device in the '569 patent may give a false indication of the authenticity of a counterfeit currency when a pencil line is drawn on the currency surface at the normal thread location.
Further U.S. Pat. No. 5,151,607 discloses a verification device comprising the optical means of the '569 patent in combination with a magnetic detector. magnetic reader, or non-ferrous metal detector. The latter detectors determine the presence of the security thread, while the optical means determines whether the thread is properly within the currency or improperly disposed on either surface. However, no known device which effectively verifies the presence and authenticity of the aforementioned “solid” security thread has been provided.
Another way may to protect high-security documents against forgery would be putting a security thread through the documents. A security thread generally consists of a thin ribbon which runs through the document. Some security threads have engravings in them (for example, the denomination of the banknote). The threads can be completely invisible or they can appear to weave in and out of the paper. Threads can also be integrated with other security features like micro-printing or fluorescence to provide additional security. Along with the security threads are also involved methods of its verification.
Many security threads have the ability to change color when viewed at different angles. Some threads use color shifting inks or flakes to create these effects using interference stacks or liquid crystal technology. More secure authentication features can be produced by employing optically variable structures, such as diffraction gratings, which change in appearance as the observer changes their angle of view relative to the structure. However, conventional “gratings” produce undesirable “RAINBOW” effect referring to variety of colors produced simultaneously thus limiting their applicability.
W02012019226 discloses a method wherein the device has slots that are separated by non-conductive regions and overlapped in dielectric material. Zero order diffraction network elements are modulated in a region in a manner that colored image is visible for a person viewing the device. The person viewing the device observes variable optical effect when the device is pivoted around an axis perpendicular to a plane. The person viewing the device observes another variable optical effect when the device is located under the polarized light and pivoted around another axis perpendicular or parallel to the plane. However the method described above doesn't provide ways to attain a large color contrast using gratings and methods to eliminate the rainbow effects produced by the diffraction grating.
W02011116425 discloses a security document comprising a substrate provided with an integral security device formed on the substrate, wherein the security device comprises an image layer and a focusing layer, the image layer including a plurality of embossed relief formations in a first radiation curable ink layer on a first surface of the document, the focusing layer including a plurality of embossed focusing element relief formations in a second radiation curable ink layer on a second surface, wherein the total thickness of the document falls substantially within the range from 60 to 140μ and said first and second surfaces are separated by a predetermined distance greater than 50μ to produce a visible optical effect when viewing the image layer through the focusing layer. However, the realization of the structure mentioned above requires extensive fabrication steps especially when one considers the use of subwavelength diffractive elements. Further, no design methodology has been presented as to how to optimize the diffractive element in order to achieve the desired color contrast.
To overcome the problems of the prior art as briefly mentioned above, inventors have come up with the current invention which by-passes the need of exhaustive verification devices and predicting the authenticity of currency notes with naked eyes.
The objective of the current invention is to design a security feature that provides an easily distinguishable color contrast (red/green or red/blue) using novel high index contrast subwavelength gratings. This method gives the advantage of higher and easily customizable color contrast to better distinguish between a fake and an authentic banknote which would appear red when viewed normally and blue/green when viewed at glancing angles. The use of asymmetric gratings can provide an additional visual effect: the security thread appears in a different color when viewed at glancing angle from different sides. Polarization effects can also be incorporated using high-contrast grating technology to provide additional security. The mechanism proposed here requires specific variables which make it resource intensive and hence is not easily replicable by the counterfeiters, making it relatively “copy-proof”.
The usage of the method is not restricted to currency notes but may also be used in identity card, passport, credit card, check, driving license, security certificate, train ticket, flight ticket, share certificate, entrance ticket, birth certificate, marriage certificate, death certificate and transcript etc to ensure authenticity.
Also the high-contrast grating disclosed here finds various applications in the field of optoelectronic devices and components such as broadband/narrowband reflectors, high-Q filters and resonators, multi-wavelength/tunable lasers, low-loss hollow core waveguides, light concentrators etc.